1. Field of the Invention
Coccidiosis, an intestinal disorder of poultry, causes an assortment of problems in the infected host. These problems range from poor feed conversion ratios in light infections to acute death in heavier infections. The disease has been estimated to cost U.S. broiler producers $300 million per year, due in part to unrealized weight gains, loss of skin pigmentation, and poor feed utilization, and in part to the cost of anticoccidial drugs.
Coccidiosis is caused by protozoans belonging to the genus Eimeria. The members of this genus have a complicated life cycle which consists of both asexual and sexual stages. The cycle is initiated when birds ingest sporulated oocysts (generally associated with fecal material). These oocysts contain invasive asexual sporozoites which are released into the bird's digestive tract. The sporozoites invade epithelial cells and develop into multinucleate structures called schizonts. Each schizont matures and releases numerous invasive asexual structures, known as merozoites, into the bird's digestive tract, where they in turn invade other epithelial cells. The sexual stage of the coccidiosis life cycle is initiated when merozoites differentiate into gametocytes. The developing asexual and/or sexual stages produce the pathological digestive tract lesions characteristic of coccidiosis. Gametocytes then fuse and the fertilization products, called oocysts, are released in the feces. The formation of oocysts completes the parasite's life cycle.
Infection by protozoans of the genus Eimeria can be alleviated, and even prevented, by the administration of anticoccidial agents. However, drug-resistant strains arise at a frequent rate and the cost of development of anticoccidials is quite high. Chickens can be vaccinated against the disease by infection with live attenuated strains of Eimeria or with nonliving parasite material. However, there is an appreciable disease effect using the former approach and a prohibitive amount of material would be required to make the latter useful on a large-scale basis. Furthermore, protection with the latter is far from complete. An alternative solution would be to produce, by genetic engineering, the protective antigens of Eimeria parasites. Once developed, these immunogens could be produced in a prokaryotic or even eukaryotic culture system in an unlimited supply and used to vaccinate chickens against subsequent disease.
2. Description of the Prior Art
Immune responses are mediated by two different effector mechanisms. One mechanism, which involves the production of antibodies by lymphoid tissue, is termed "humoral immunity." The other, which involves the activation of white blood cells such as T-lymphocytes previously sensitized to the immunogen, is termed "cell-mediated immunity." See Immunology: Basic Processes, 2nd ed., page 12, J. Bellanti (1985).
PCT Application Publication No. WO 86/00528 to Anderson et al., titled "Cloned Gene and Method for Making and Using the Same," discloses cloned genes which code for antigenic proteins of Eimeria species. The procedure taught by this application for screening transformed cells to identify these DNA sequences involves the use of polyvalent chicken antiserum obtained from chickens previously infected with Eimeria tenella. See Id. at 9, 41-43, and 44. Chicken antiserum, however, reflects only the humoral immune response of the bird to the antigen to which the bird has been exposed, and does not reflect the bird's cell-mediated immune response. Accordingly, the Anderson patent teaches how to obtain DNA sequences which code for the production of antigens which evoke a humoral immune response. While an important contribution to the art, Anderson does not address whether these same antigens evoke a cell-mediated immune response, and--if not--how such antigens might be obtained. Nor does it address the relative contribution of the cell-mediated immune response and the humoral immune response to the integrated immune response of birds to avian coccidiosis.
Accordingly, an object of the present invention is to provide a means for producing DNA sequences which code for antigens which evoke a cell-mediated immune response to avian coccidiosis.
An additional important object of the present invention is to provide a means for identifying DNA sequences which code for antigens which evoke a cell-mediated immune response to avian coccidiosis and which may not otherwise be identified by prior art procedures.
A further object of the present invention is to provide cloned genes which code for Eimeria sporozoite or merozoite cell-surface antigens, particularly Eimeria acervulina cell-surface antigens.
A still further object of the present invention is to provide transformed host cells which produce antigens which evoke a cell-mediated immune response to avian coccidiosis.
Still further objects of the present invention are to provide methods and vaccines useful for protecting birds against avian coccidiosis.
Other objects and advantages of this invention will become readily apparent from the ensuing description.